The invention concerns a retaining material, particularly for removing arsenic and/or phosphorus from a petroleum charge, its method of preparation and its use.
According to their source, crude petroleums may contain traces of many metallic compounds, generally in the form of organo metallic complexes.
These organo metallic compounds are poisons to the catalysts used in petroleum conversion processes.
The organo metallic compounds are chiefly contained in heavy cuts emanating from distillation of crude petroleum. More particularly, heavy cuts from distillation under vacuum contain many metals such as nickel, vanadium or arsenic and non-metallic elements such as phosphorus. However, organic phosphorus compounds will also be described as organo metallic by extension, as the phosphorus is contained in them in the cationic state. These heavy cuts normally undergo thermal or catalytic cracking treatments to convert them to lighter, unsaturated hydrocarbon cuts which can be upgraded better.
Metals such as nickel and vanadium are not generally contained in the effluents. On the other hand arsenic, which is much more able to form volatile compounds, is contained in lighter cuts such as C.sub.2 and C.sub.3 cuts containing ethylene and propylene; these are generally purified by selective hydrogenation on catalysts based on noble metals, which are respectively poisoned by arsenic reducing compounds such as arsine and methylarsine.
French Patent 2619120 teaches that these reducing compounds can be absorbed, in the gas phase or in the liquid phase, on lead oxide deposited e.g. on alumina.
On the other hand, if heavier cuts such as petrol or naphtha have to be treated, the arsines normally present have a boiling point higher than methylarsine and thus contain one or more hydrocarbon radicals in their molecule. Such compounds have a much weaker reducing power, and reactions for reducing lead oxide are not completed. Absorption materials containing metal oxides such as lead oxide are then ineffective on liquid hydrocarbon cuts of this type. It is the same with organic phosphorus compounds, which cannot be eliminated in this way. Patent FR 2617497 therefore proposes the use of an absorbent material, comprising nickel deposited on a carrier such as silica, magnesia or alumina. However, these retaining materials are found to have a limited effect and limited stability when they are regenerated.
Finally, patents GB-A-1144 497, EP-A-0239 687 and FR-A-2619 121 illustrate the state of the art.